Gas sensors are widely used to detect leaks of inflammable gases, such as hydrogen and ethanol, etc., and have recently been used for detection of aromatic components and toxic gases in food and environment-related fields. Therefore, as gas-sensing properties of gas sensors, elavation of detectability of smaller amounts of minor components from the gases in question, improvement of gas selectivity, expansion of the types of gases to be detected and the like are requested.
Moreover, in the inflammable gas sensors used for city gas alarm devices and the like, which hold a big share of the gas sensors, there is a need for sensors capable of detecting city gas, such as methane gas and the like, and simultaneously of detecting incompletely burned gas for preventing carbon monoxide poisoning and the like. As the hybrid-type gas leak alarms for city gas and incompletely burned gases developed thus far, multidevice-type alarms with a sensor for methane gas and a sensor for carbon monoxide are generally used and each of these sensors are controlled with a computer, and therefore, there is a limit to the extent of the alarms to be miniaturized. A so-called 1-element type sensor capable of differentiating and detecting city gas and incompletely burned gases with 1 sensor by changing operating conditions, including operating temperature and the like of the sensor, has not been marketed yet and therefore, there is still room for improvement [for instance, K. Tsuda, "The Most Recent Technological Trends in Inflammable Gas Sensors", Nyu Seramikkusu (New ceramics), No. 12, p27-32, (1997)].
Methane gas, which accounts for approximately 90% of city gas, is one of the inflammable gases with low chemical activity and it is difficult to detect only methane with a semiconductor gas sensor. As a countermeasure to the above-mentioned problem, by using a method of adding a sensitizer (catalyst) such as Pd or the like, or a method of using needle-shaped semiconductor particles with large surface area which can increase reaction space and improve gas diffusivity, selectivity of the gas sensor to methane, which is relatively non-inflammable, can be improved, and, for example, gas sensitivity (temperature of 300 to 500.degree. C.) of 10 to 20 to 0.5% methane is obtained [for instance, N. Miura et al., "Surface Modification in Semiconductor Gas Sensors", Hyomen (Surface), vol. 28, No. 1, p15-25, (1990), H. Ohnishi et al., "Sensing properties of highly selective methane sensor based on SnO.sub.2 thin film", Nihon Seramikkusu Kyokai Shuki Shinpojiumu Koen Yokoshu (Japanese Ceramic Association, Autumn Symposium Summary for Lecture), p240, (1996)]. Besides, the sensitivity is defined as a ratio of the electrical resistance of a sensor in air (Ra) to that in a sample gas (Rg).
With respect to gas sensors for incompletely burned gases, such as carbon monoxide and the like, which require detectability of even smaller amounts of minor components of the gases for preventing poisoning compared with that for city gases, and it is reported that the temperature at which maximum sensitivity is obtained shifts to the low-temperature side and sensitivity of the sensors is improved by adding Pd and Au due to the effects of these sensitizers. For instance, gas sensitivity (Ra/Rg) of 10 to 20 to 0.02% carbon monoxide is obtained when Pd is added [for instance, N. Yamazoe et al., "The Role of Metal Microparticles in Semiconductor Gas Sensors", Hyomen (Surface), vol. 27, No. 6, p499-507, (1989); M. Haruta, "Gold Super-Microparticle Solic Oxides as Room-temperature Catalysts", Hyomen (Surface), vol. 28, No. 5, p333-342, (1990)].
Methods have been studied whereby a layer-built structure having the sensor layer surface covered with a catalyst layer or filter layer is used in order to expend or block components other than the desired gases as a purpose for improving selectivity of specific gases in a mixture of several gases. However, problems with gas diffusivity have been pointed out in this method and further it is reported that it takes longer periods until electrical symbols stabilize compared with the case where a coating layer is not used [for instance, C. D. Feng et al., "Effect of Gas Diffusion Process on Sensing Properties of SnO.sub.2 Thin Film Sensors in a SiO.sub.2 /SnO.sub.2 Layer-Built Structure Fabricated by Sol-Gel Process," J. Electrochem. Soc., Vol. 141, p220-225 (1994)]. Moreover, the fuction of the coating layers have been tested hitherto as catalysts or filters, etc., and coating layers have not been used as separate gas sensors.
In order to solve the disadvantages of the above-mentioned type of conventional semiconductor gas sensors, the present invention has been developed and this invention provides a semiconductor gas sensor element which is useful for obtaining a sensor capable of detecting several gases with good selectivity by 1 element and has a layer-built structure where the surface of a methane sensor is covered with a carbon monoxide sensor.
The inventors have performed intense research to develop a 1 element type semiconductor gas sensor capable of detecting methane and carbon monoxide with good selectivity and as a result, they have focused on the fact that since methane has a low chemical activity, somewhat high temperature is needed to detect the gas, and conversely, it is possible to shift sensitivity to carbon monoxide higher on the low-temperature side and lower at a high temperature by using a sensitizer. They also have studied application of a coating technology in order to control the morphology of the semiconductor particles of the methane sensor in view of detecting methane gas with low activity and further detecting several gases. That is, the inventors have made a layer-built structure where a carbon monoxide gas sensor is coated on the surface of a methane gas sensor to give the carbon gas sensor layer the ability to function as a catalyst film that blocks carbon monoxide, which prevents the detection of methane. They have discovered that their purpose can be accomplished based on the improvement in gas diffusivity that is obtained by using a porous layer-built structure whose sensor layers are formed with rosary beads and the like. They have successfully completed the present invention based on these knowledge.